In recent years, devices applying wireless techniques, such as wireless LANs complying with IEEE 802.11a/b/g standards, and Bluetooth, have been rapidly spreading. IEEE 802.11a and IEEE 802.11g specified the data transmission rate of 54 Mbps, and recently, active researches and developments have been done on wireless schemes for achieving higher transmission rates.
As one of techniques for increasing transmission rates of wireless communication systems, a MIMO (Multi-Input Multi-Output) communication system has received wide attention. This is a technique for increasing transmission capacity and improving communication speed by providing both the transmitter and the receiver with multiple antenna elements and having transmission paths spatially multiplexed. This technique is essential not only for wireless LANs, but also for next-generation wireless communication systems such as mobile phone communication systems and IEEE 802.16e (WiMAX).
In the MIMO communication scheme, a transmitter divides and sends transmitting data through multiple antenna elements, the data is transmitted over multiple virtual MIMO channels, and a receiver receives signals through multiple antenna elements and processes the signals to obtain received data. Generally, a wireless device using the MIMO communication scheme is provided with multiple omnidirectional antenna elements such as dipole antennas or sleeve antennas. In this case, there is a problem of degradation in transmission quality caused by increases in the correlations between antenna elements, unless addressing this situation by, e.g., sufficiently separating the antenna elements from one another, or tilting the respective antenna elements in different directions to make a combination of different polarizations.
As a conventional technique available for solving the above problem, for example, there is an array antenna apparatus of Patent Literature 1, which is an adaptive directional antenna. The array antenna apparatus of Patent Literature 1 includes three printed wiring boards arranged so as to surround a half-wave dipole antenna mounted vertically on a dielectric supporting substrate. The half-wave dipole antenna is supplied with a radio frequency signal through a balanced feeder cable. Moreover, on the back side of each printed wiring board, two sets of parasitic elements are provided in parallel with each other, each set including two printed antenna elements (elements each made of a conductor pattern). In each parasitic element, the two printed antenna elements oppose to each other with a space therebetween. A through-hole conductor is provided at one end of each printed antenna element opposing to the other printed antenna element, and is connected to an electrode terminal on the front side of the printed wiring board. In each parasitic element, a variable-capacitance diode is mounted between the two electrode terminals, and these electrode terminals are further connected to a pair of cables through high value resistors for blocking high frequencies, and the pair of cables are connected to bias voltage supply terminals DC+ and DC− of a controller (not shown) for controlling the directivity of the array antenna apparatus. By changing bias voltages supplied from the controller, the respective reactance values of the variable-capacitance diodes connected to the parasitic elements change. In this manner, the electrical length of each parasitic element is changed as compared to that of the half-wave dipole antenna, thus changing the horizontal directivity of the array antenna apparatus.
It is possible to reduce distances between antennas by using, as MIMO communication antennas, adaptive directional antennas such as the array antenna apparatus of Patent Literature 1, and setting the respective antennas' directivities so as to avoid correlations between the antennas.
Furthermore, by using adaptive directional antennas for MIMO communication, two advantages can be expected as follows. As a first advantage, in the case of a low electric field level at the receiver side, it is possible to direct a beam in a direction of arrival of a strong radio wave, thus receiving at a stable electric field level. As a second advantage, when fading occurs due to reflected waves from a wall or ceiling, one antenna receives a direct wave and the other antennas receive the reflected waves with long delay time, thus achieving more effective MIMO wireless communication.